The Big Dig is a side quest that you can pick up in Goodneighbor during your travels in Fallout 4. It is a quest that impacts a specific companion and can potentially make them impossible to recruit.
BETRAY OR STAY WITH BOBBI-The Quest "The Big Dig" Unique MiniGun-THE ASHMAKER
We'll take you through The Big Dig in Fallout 4 so you can make the best decision when it comes time to choose a side.
To begin the quest, head to the town of Goodneighbor. When you enter the town, continue straight until you reach an alley on your right. Hanging lights decorate the alley, so it's hard to miss.
Continue through the alley and you will come to a door with a slot in the front. Approach the door and the slot will open, revealing a Ghoul named Bobbi No-Nose. She will ask if you are looking for a job. Ask her any further questions or agree directly to begin the quest.